05000416/LER-2011-001
| Grand Gulf Nuclear Station, Unit 1 | |
| Event date: | 03-19-2011 |
|---|---|
| Report date: | 05-12-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 4162011001R00 - NRC Website | |
A Reportable Occurrence The High Pressure Core Spray (HPCS) system [BG] was declared inoperable due to discovery of HPCS Minimum Flow Valve (1E22F012) breaker (52-170109) instantaneous overcurrent trip set point to be out of tolerance, resulting in a single train failure.
B. Initial Conditions The reactor was in OPERATIONAL MODE 1 with reactor power at approximately 96 percent.
There were no additional inoperable structures, systems, or components that contributed to this event.
C. Description of Occurrence On March 18, 2011 Instrument and Control (I&C) technicians were in the process of performing Surveillance 06-IC-1E22-Q-0004, HPCS System Flow Rate Low (Bypass) Functional Test. The HPCS system was inoperable at this time for the scheduled maintenance. The 18,C surveillance required installing a current calibrator into the control circuit for 1E22F012, HPCS Minimum Flow Valve. While connected, the current calibrator failed due to loss of battery power. This caused a repeated cycling of the HPCS Minimum Flow Valve (1E22F012). 1E22F012 power was lost and it was identified that 1E22F012 feeder breaker, 52-170109 had tripped. Breaker 52-170109 was tested, the current calibrator was replaced and the surveillance was completed successfully. On March 18, 2011, at 1127 the HPCS system was declared operable.
During the debrief of the electrical technician who had completed the retest and reinstalled the breaker, it was discovered that the breaker testing was inadequate. The retest instructions specified in the work order were not clear as to the intent and did not adequately check the breaker overcurrent instantaneous trip setpoint. Preparations were started to retest breaker 52-17019. On March 19, 2011, at 0330 the HPCS system was declared inoperable to allow the breaker to be removed for testing, On March 19, 2011, at 2236 breaker 52-170109 was tested and it tripped at an instantaneous setting of 58 amps. The breaker tripping at 58 amps which is below the manufacturer's tolerance for the existing setting of 107 amps. This rendered the single train system incapable of fulfilling its safety function.
The HPCS system being incapable of fulfilling its safety function met the criteria of 10 CFR 50.72(b)(3)(v)(D) for an 8 hour9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> Nuclear Regulatory Commission (NRC) required report, The time of discovery was March 19, 2011 at 2236.
On March 20, 2011, at 0700 following successful post maintenance testing, H PCS was declared operable.
D. Cause of Occurrence The repeated cycling of the valve open and closed and the resulting surge currents created excessive heat in the circuit breaker instantaneous trip and overload circuits. The condition caused the failure of the motor overload relay circuit to trip and also resulted in breaker setpoint drift. This resulted in the single train system being inoperable.
The root cause of the failure was a power failure in the current calibrator test equipment used in performance of the HPCS System Flow Rate Low (Bypass) Functional Test. The loss of power was due to exhaustion of the batteries powering the unit.
The contributing factor was the lack of procedural guidance requiring new batteries or use of the AC power source.
Corrective Actions
Immediate Actions
1) A replacement breaker was obtained, tested and installed.
2) Valve 1E22F012 was retested after breaker replacement.
The corrective actions for the contributing cause are:
- Procedure 06-IC-1E22-Q-0004 was revised to require either use of new batteries or NC power when using the current calibrators
- Clarify what value of milliamp direct current (madc) to input when using the current calibrator The corrective actions were completed as required by the GGNS Corrective Action Program under CR-GGN-2011-1902.
E Safety Assessment According to the GGNS Technical Specification Bases, the High Pressure Core Spray (HPCS) is part of the Emergency Core Cooling Systems (ECCS). The ECCS is designed, in conjunction with the primary and secondary containment, to limit the release of radioactive materials to the environment following a loss of coolant accident (LOCA). The ECCS uses two independent methods (flooding and spraying) to cool the core during a LOCA. The ECCS network is composed of the High Pressure Core Spray (HPCS) System, the Low Pressure Core Spray (LPCS) System, and the low pressure coolant injection (LPCI) mode of the Residual Heat Removal (RHR) System.
The ECCS also consists of the Automatic Depressurization System (ADS). The suppression pool provides the required source of water for the ECCS. The design basis of the HPCS System is to provide core cooling over a wide range of RPV pressures (0 psid to 1177 psid, vessel to suction source). Upon receipt of an initiation signal, the HPCS pump automatically starts approximately 10 seconds after AC power is available and valves in the flow path begin to open. Since the HPCS System is designed to operate over the full range of expected RPV pressures, HPCS flow begins as soon as the necessary valves are open. The TS Bases also states that the ECCS System satisfies Criterion 3 of the NRC Policy Statement. This means that it is considered to be a part of the primary success path which functions to mitigate a design basis accident or transient that either assumes the failure of or presents a challenge to the integrity of a fission product barrier.
F. Safety Assessment (continued) The HPCS system was inoperable and incapable of performing its design function for approximately 15.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />. The GGNS UFSAR states that in the event of a HPCS system failure such that HPCS cannot maintain the reactor water level, the automatic depressurization system, which is independent of any other ECCS, reduces the reactor pressure so that flow from LPCI and LPCS systems enters the reactor vessel in time to cool the core and limit fuel cladding temperature.
Additionally, high pressure makeup was available from the reactor core isolation cooling (RCIC) system. Two of three divisions of emergency core cooling systems (ECCS) are required for the GGNS loss of coolant accident analyses. While HPCS was out of service, Division 1 and Division 2 ECCS systems and the automatic depressurization system were operable, and would have met the ECCS performance criteria of 10CFR50.46. The out of service time was maintained within the limits of GGNS technical specifications, thereby minimizing any safety significance of this event.
G. Additional Information
Previous Occurrences - There has not been any occurrence of an HPCS event or condition in the past two years at Grand Gulf Nuclear Station involving reportability under 10CFR50.73(a)(2)(D) or involving these same conditions. The corrective action response addressed the extent of the cause of this event. HPCS is a single train system.